Bull Lake Research Articles

A cosmogenic 10Be moraine chronology of arid, alpine Late Pleistocene glaciation in the Pioneer Mountains of Montana, USA

We test the hypothesis that glacier systems, located in continental regions proximal to the Laurentide Ice Sheet (LIS), had local ice maxima considerably earlier than the LIS maximum and thus before the insolation minima at ∼21 ka. Ranges located in the northwest US exhibit earlier deglaciation timing between ∼23 and 22 ka, except for the Yellowstone region where younger time-transgressive ages complicate regional interpretations and the northern Montana ice cap where late glacial ages have recently been produced. Constraining the glacial history of more ice sheet-proximal alpine glaciers provides insight into whether the contrasting maximum-ice times in the northern Rocky Mountains were caused by regional climatic differences, such as anticyclonic wind patterns driven by the presence of the LIS. In the Pioneer Mountains of Montana, we measured in situ cosmogenic 10Be in 35 boulders on moraines marking the maximum Late Pleistocene positions of alpine glaciers from three valleys. The 10Be samples produced a range of ages, spanning pre Bull Lake to the last glaciation (i.e., Pinedale/Marine Isotope Stage (MIS) 2). We find an average exposure age for initial deglaciation of 18.2 ± 0.9 during the local Last Glacial Maximum, indicative of synchronous retreat in the Pioneer Mountains. The similarity of initial deglaciation timing of the Pioneer Mountain glaciers with the northwestern Yellowstone glacial system and northern MT ice cap suggests that topography more proximal to the LIS margin maintained full ice extent longer. Our findings, in context of previous work, suggest that in the case of the Pioneer Mountains their more proximal location to the ice margin may have delayed onset of deglaciation by greater exposure to local cooling from katabatic winds and/or additional moisture sourced from large ice-marginal glacial lakes, hence the lack of earlier deglacial ages like those found further to the west and east of the northern Rocky Mountain cordillera.

Genesis of Sublayer, Footwall Breccia, and Associated Ni-Cu-Platinum Group Element Mineralization in the Sudbury Igneous Complex

Abstract The Ni-Cu-platinum group element (PGE) sulfide mineralization in the 1.85 Ga Sudbury impact structure occurs in two discrete environments: (1) mineralization along or near the basal contact, including disseminated to semimassive mineralization in an inclusion-rich sublayer, semimassive to fragmental mineralization in the underlying anatectic footwall breccia, and veins and disseminations in the underlying pseudotachylitic Sudbury breccia, and (2) disseminated to semimassive mineralization in the inclusion-bearing quartz diorite phase of radial and concentric offset dikes that extend up to 20 km from the basal contact. The sublayer, inclusion quartz diorite, and footwall breccia all contain abundant mafic-ultramafic inclusions—local xenoliths derived from nearby country rocks, exotic xenoliths derived from unexposed upper-middle crustal target rocks and, locally, anteliths derived from an olivine melanoritic early border phase of the Sudbury Igneous Complex—but the quartz diorite margins of offset dikes contain very few local inclusions and no exotic ultramafic inclusions. The similar inclusion populations indicate a genetic relationship between the sublayer and inclusion quartz diorite and interaction between the sublayer and footwall breccia. But the sublayer is characterized by a cumulate noritic matrix, the inclusion quartz diorite by a noncumulate quartz dioritic matrix, and the footwall breccia by an anatectic felsic-intermediate matrix. The overlying main mass norite is very homogeneous in terms of Hf isotopes, indicating that the impact melt sheet was well mixed, but ores, sublayer, inclusion quartz diorite, and to a lesser degree overlying main mass norites vary widely in their Pb-S-(Os) isotope compositions. The majority of mafic-ultramafic inclusions, except for anteliths, contain no sulfide and exhibit no signature of Ni-Cu-PGE depletion caused by prior sulfide saturation, which indicates that the association between mafic-ultramafic inclusions and Ni-Cu-PGE mineralization is attributable primarily to the refractory nature of the inclusions and to a lesser degree their similar hydrodynamic behavior as the sulfide melt and, secondarily, to derivation of sulfides from the same sources as the inclusions. The very large Sudbury impact event volatilized much of the Pb-S-Zn-Cd-Se-Bi and significant amounts of Sb-Ag-Cu-Au-As from the target rocks. It generated large amounts of superheated impact melt and only minor fragmental debris during the compression and excavation stage, which is when the inclusion- and sulfide-poor marginal phase of the offset dikes was emplaced. Large amounts of debris, including sulfide-bearing Huronian basalts, Nipissing and East Bull Lake intrusions, and Archean mafic gneisses, were generated during isostatic rebound, formation of a central uplift, and collapse of the central uplift and crater walls, which is when the inclusion- and sulfide-bearing internal phase of the offset dikes was emplaced. Convective and gravity flow aided horizontal transport of residual exotic inclusions, local inclusions, and sulfide xenomelts into embayments and funnels to form the protosublayer. Olivine-saturated melts, generated by thermomechanical erosion of local olivine-bearing country rocks, locally crystallized an olivine melanoritic early border phase of the main mass, which was disrupted and preserved in the funnels of some North Range offset dikes. Continued thermomechanical erosion of country rocks enlarged dike funnels and exploited other fractures to generate footwall embayments and significant geochemical and Pb-S-(Os) isotope heterogeneities. As the rate of thermomechanical erosion decreased and the rate of heat conducted into the footwall rocks increased, contact ores and some offset ores fractionally crystallized to form residual melts that generated footwall vein systems.

Evidence of magmatism and rifting in the southern superior craton from the Temagami geophysical anomaly

The Temagami Geophysical Anomaly (TGA) is situated in the Paleoproterozoic Huronian Supergroup near the southern margin of Archean Superior craton, along strike from the Temagami Greenstone Belt, 50 km northeast of world-class metal endowment in the Sudbury Igneous Complex. It has been attributed to coincidental geophysical responses from Neoarchean iron formation and a Paleoproterozoic mafic–ultramafic intrusion. The TGA is investigated using integrated magnetotelluric, magnetic, gravity and seismic reflection datasets to further define the geometry of its sources; to define structures in overlying Huronian Supergroup rocks; and to examine the geometrical relationship of Archean basement structure, the Proterozoic mafic–ultramafic intrusion, and Proterozoic rift structures.Geophysical results reveal a strong conductor, interpreted to be iron formation, extending downwards from the Archean basement surface and enveloping the upper northern margin of a large magnetic, dense body at 5 km depth. The geometry of the features indicates mafic–ultramafic intrusion adjacent to greenstone rocks. Alignment of the long-axis of the intrusion with the greenstone belt suggests control on the intrusion process by the Neoarchean structures. The 60x10x10 km TGA intrusion lies within the mantle-plume, rift-related Huron-Nipissing magmatic belt and interpreted to be a member of the 2.491–2.475 Ga East Bull Lake intrusive suite. Based on this genetic relationship, the intrusion probably contributes significantly to Ni-Cu-PGE endowment of the Sudbury region. The spatial correlation of the intrusion with an overlying 20 km wide, 4.5 km deep, fault-bounded rift basin is attributed to crustal subsidence triggered by the intrusion. Seismic reflection and magnetotelluric results show younger Huronian Supergroup rocks record the transition to sedimentation on a laterally-extensive passive margin. An electrically-anisotropic, moderately-conductive layer in the Gowganda Formation is related to alteration during intrusion of the 2.200 Nipissing Diabase.

Calibrating a long-term meteoric <sup>10</sup>Be delivery rate into eroding western US glacial deposits by comparing meteoric and in situ produced <sup>10</sup>Be depth profiles

Abstract. Meteoric 10Be (10Bemet) concentrations in soil profiles have great potential as a geochronometer and a tracer of Earth surface processes, particularly in fine-grained soils lacking quartz that would preclude the use of in situ produced 10Be (10Bein situ). One prerequisite for using this technique for accurately calculating rates and dates is constraining the delivery, or flux, of 10Bemet to a site. However, few studies to date have quantified long-term (i.e., millennial) delivery rates, and none have determined a delivery rate for an eroding soil. In this study, we compared existing concentrations of 10Bein situ with new measurements of 10Bemet in eroding soils sampled from the same depth profiles to calibrate a long-term 10Bemet delivery rate. We did so on the Pinedale (∼ 21–25 kyr) and Bull Lake (∼ 140 kyr) glacial moraines at Fremont Lake, Wyoming (USA), where age, grain sizes, weathering indices, and soil properties are known, as are erosion and denudation rates calculated from 10Bein situ. After ensuring sufficient beryllium retention in each profile, solving for the delivery rate of 10Bemet, and normalizing for paleomagnetic and solar intensity variations over the Holocene, we calculate 10Bemet fluxes of 1.46 (±0.20) × 106 atoms cm−2 yr−1 and 1.30 (±0.48) × 106 atoms cm−2 yr−1 to the Pinedale and Bull Lake moraines, respectively, and compare these values to two widely used 10Bemet delivery rate estimation methods that substantially differ for this site. Accurately estimating the 10Bemet flux using these methods requires a consideration of spatial scale and temporally varying parameters (i.e., paleomagnetic field intensity, solar modulation) to ensure the most realistic estimates of 10Bemet-derived erosion rates in future studies.

Cosmogenic 10Be exposure dating of Bull Lake and Pinedale moraine sequences in the upper Arkansas River valley, Colorado Rocky Mountains, USA

AbstractMany formerly glaciated valleys in the western United States preserve detailed glacial features that span the penultimate glaciation through the last deglaciation; however, numerical age control is limited in many of these systems. We report 35 new cosmogenic 10Be surface exposure ages of moraine boulders in the Sawatch Range, Colorado. Eight ages suggest Bull Lake moraines in Lake Creek (range: 132–120 ka, n = 4) and Clear Creek (range: 187–133 ka, n = 4) valleys may correlate with Marine Isotope Stage 6. In Lake Creek valley, 22 10Be ages from Pinedale end moraines average 20.6 ± 0.6 ka, and 5 10Be ages from a recessional moraine average 15.6 ± 0.7 ka, indicating that glaciers occupied two extended positions at ~21–20 and ~16 ka. The glacial extent dated to ~16 ka was nearly as great as that of the earlier glacial phase, suggesting that climate conditions in the Colorado Rocky Mountains at this time were similar to those of the last glacial maximum. Combining these moraine ages with seven previously published 10Be ages from cirque and valley-bottom bedrock reveals that the Lake Creek paleoglacier lost 82% of its full glacial length in ~1.5 ka and was completely deglaciated by ~14 ka.

Mafic intrusions in the footwall of the Sudbury Igneous Complex: Origin of the Sudbury impact melt sheet and its associated ore deposits

The majority of the mafic intrusions and volcanic rocks in the footwall of the Sudbury Igneous Complex (SIC), which is believed to have formed from a melt sheet produced by the impact of a very large bolide, belong to one of two large mafic igneous suites, viz: the ∼2450 Ma East Bull Lake (EBL) suite of intrusions or the ∼2200 Ma Nipissing Diabase suite of intrusions. This has been established by U-Pb LA-ICP-MS dating of zircons from the newly recognized Frood intrusion, which hosts the Frood and Stobie ore bodies, and two of the Sudbury Gabbro intrusions, and comparison of the geochemistry of these intrusions with that of the EBL intrusive suite and the Nipissing Diabase suite. The age of the Frood intrusion (2421 ± 32 Ma) falls within error of other EBL-type intrusions whereas the ages of the Nairn (2203 ± 11 Ma) and Totten (2168 ± 11 Ma) intrusions fall within error of published ages for the Nipissing Diabase. Moreover, the primitive mantle-normalized patterns of the petrogenetic trace elements on extended spidergrams of the Nairn and Totten Sudbury Gabbros are similar to those of the Nipissing Diabase as are those of the Frood intrusion to other members of the EBL intrusive suite.The Ni-Cu-PGE sulfide deposits at Sudbury are associated with the lower contact of the SIC, where a discontinuous inclusion-rich norite contains much of the mineralization at the contact, and an inclusion-bearing quartz diorite is directly associated with the mineralization of the radial and concentric offset dykes. The Sublayer rocks and the overlying unit of melanocratic norites provide strong evidence for a relationship between the ore deposits and both inclusions and melts of mafic-ultramafic target rocks. The most common pre-impact mafic-ultramafic target rocks are intrusions of the EBL and Nipissing Diabase suites, although there are also Archean-aged mafic-ultramafic enclaves in the gneisses beneath the North Range of the SIC. The Totten orebody in the Worthington Offset and the Kelly Lake ore body in the Copper Cliff Offset are directly associated with footwall rocks comprising Sudbury Gabbro and Nipissing Gabbro, respectively, whereas the Frood-Stobie Offset and the associated Frood and Stobie ore deposits are sandwiched in a breccia belt containing large rafts of amphibole megacrystic gabbro belonging to the extensively broken-up Frood intrusion. Olivine-bearing mafic and ultramafic rocks, some of which exhibit shock metamorphic features, but others do not, are common inclusion types and occur in diverse Sublayer environments. The unshocked melanorite and olivine melanorite inclusions share common geochemical characteristicsde, indicating that they were derived from a similar source. Their geochemistry is also similar to that of the Frood Intrusion as well as the East Bull Lake intrusion; this indicates that they may have been sourced from East Bull Lake-type intrusions that were incorporated into the melt sheet at the time of melt generation.Mass balance calculations indicate that the felsic country rocks which account for 77% of the total volume of the SIC and contain 18 ppm Ni and 21 ppm Cu together with the sulfide-poor mafic country rocks which account for 23% of the SIC and contain 133 ppm Ni and 92 ppm Cu have too little Ni and Cu to account for the Ni and Cu contents of the quartz diorite, the composition of which provides an indication of that of the initial melt sheet.Calculations using the R-factor equation of Campbell and Naldrett (1979) indicate that the magmas that formed the norites and the ore deposits had 137 ppm Ni and 121 ppm Cu. Mass balance calculations have been used to determine if the country rocks have sufficient Ni and Cu in them to generate the ore deposits. The weighted median Ni content of the sulfide poor (<200 ppm Cu) mafic country rocks is 133 ppm and hence very similar to that of the ore-forming magma. However, the weighted median Cu contents of these rocks is only 90 ppm and hence too low to provide all of the Cu required by the ore-forming magma. What the mass balance calculation shows is that a contribution of Cu (and Ni) in the ore-forming magma was derived from pre-existing Ni-Cu-PGE sulphides associated with the mafic target rocks. This contribution was diluted by the lower concentrations of Ni and Cu from the Proterozoic metasedimentary rocks and the Archean granitoids and gneisses. For this reason, a contribution from magmatic sulfides in the mafic target rocks is required, and this may have been similar to the known disseminated concentrations of Cu-PGE-Ni sulfides present in mafic intrusions in the Sudbury area.

Alluvial fan sediments and surface ages resulting from differing climatic and tectonic conditions in Star Valley, Wyoming, USA

Alluvial fans occur in all climatic and tectonic environments. However, most descriptions of fan features and sediments have focused on arid and semiarid regions. This paper describes the sediments and soil ages on 63 alluvial fans in a subhumid climate (mean annual precipitation of 45–52 cm). SSURGO soil data were merged with geologic and topographic data in the ArcMap© GIS platform to develop a series of maps showing variations across fan surfaces in sediment textures and surface stability ages. Particle size data (percent cobble and percent total gravel) from the upper 150 cm of sediments cluster into six textural groups that are aligned along a trend of decreasing gravel percentage (decreasing stream capacity) and decreasing cobble percentage (decreasing stream competency). The groups of higher cobble and gravel percentages extend from fan apex to toe in both the shallow (<50 cm depth) and deeper (>100 cm depth) layers with groups of lower cobble and gravel percentage occurring along fan margins in the shallow (<50 cm depth) layer. The thickness of B and Bt horizons from representative soil profiles were used to calculate proportional soil ages based on 10Be residence times in loessial Bt horizons and correlation of gravelly fan soils to similar gravelly soils on nearby Pinedale outwash. This approach identified six stability episodes since the end of the Bull Lake beginning at ~100 ka, ~38 ka, ~27 ka, ~16 ka, ~10 ka, and <2 ka. The proportional soil ages correlate closely with fan chronologies constrained by TL and OSL ages in nearby areas. Soil ages and sediment textural groups differed between fan populations along the mountain front with active faulting and those along the stable mountain front. Fan stability surfaces older than 16 ka are preserved only at fault termini. Holocene faulting resulted in fans with (1) coarser sediments, (2) younger sediments, (3) progressive onlap of younger sediments, (4) the absence of relic fan surfaces, landslides and colluvium at the mountain front, and (5) the onlap of alluvial fans onto the floodplain of the axial stream. Sediment pulses related to individual fault events could not be definitively identified because of concurrent climatic changes. The study demonstrates that both climatic and tectonic conditions influence sediment textures and fan surfaces ages.

Late Pleistocene glaciation in the Mosquito Range, Colorado, USA: chronology and climate

ABSTRACTNew cosmogenic 10Be surface exposure ages from 17 moraine boulders in the Mosquito Range of Colorado suggest that glaciers were at their late Pleistocene (Pinedale) maximum extent at ∼21–20 ka, and that ice recession commenced before ∼17 ka. These age limits suggest that the Pinedale Glaciation was synchronous within the Colorado Rocky Mountain region. Locally, the previous (Bull Lake) glaciation appears to have occurred no later than 117 ka, possibly ∼130 ka allowing for reasonable rock weathering rates. Temperature‐index modeling is used to determine the magnitude of temperature depression required to maintain steady‐state mass balances of seven reconstructed glaciers at their maximum extent. Assuming no significant differences in precipitation compared to modern values, mean annual temperatures were ∼8.1 and 7.5 °C lower, respectively, on the eastern and western slopes of the range with quantifiable uncertainties of + 0.8/−0.9 °C. If an average temperature depression of 7.8 °C is assumed for the entire range, precipitation differences − that today are 15–30% greater on the eastern slope due to the influence of winter/early spring snowfall − might have been enhanced. The temperature depressions inferred here are consistent with similarly derived values elsewhere in the Colorado Rockies and those inferred from regional‐scale climate modeling.

Late summer glacial meltwater contributions to Bull Lake Creek stream flow and water quality, Wind River Range, Wyoming, USA

ABSTRACTThe Wind River Range in Wyoming contains more glacial ice than any other location within the USA’s Rocky Mountain states of Colorado, Idaho, Montana, and Wyoming. Bull Lake Creek watershed in the southeast portion of the range contains five major (0.6–1.5 km2) glaciers along with numerous smaller glaciers that contribute to the Wind River. Field measurements were made of discharge from the Knife Point and Bull Lake Glaciers to determine the contribution of glacial meltwater to the river system. Water samples were collected and analyzed for stable isotopes, major ions, nutrients, and selected trace elements. Meltwater from the two glaciers contributed 13.9% to Bull Lake Creek streamflow (site BL-3), with all glaciers within the Bull Lake Creek watershed estimated to be contributing 55.6% to the streamflow of Bull Lake Creek (United States Geological Survey gage) during the August 2015 study period. Hydrogen and oxygen stable isotope analysis indicated as much as 80% of late summer discharge in the upper Bull Lake Creek watershed was attributed to glacial meltwater. This study also found that nutrients (NO3 – NO2, total P) from glacial meltwater can be a significant source of nutrient loading to Bull Lake Creek.

Revised Quaternary glacial succession and post-LGM recession, southern Wind River Range, Wyoming, USA

We present here a more complete cosmogenic chronology of Pleistocene glacial deposits for the Wind River Range, Wyoming, USA. Fifty-one new and thirty-nine re-calculated 10Be and 26Al exposure ages from Sinks and North Fork canyons, Stough Basin, Cirque of the Towers and the Temple Lake valley allow us to more tightly constrain the timing and sequence of glacial alloformations in the southern portion of the range.Moraines, diamicts and bedrock exposures here have previously been correlated with as many as five Pleistocene and four Holocene glacial events. Exposure ages from Pleistocene alloformations associated with trunk glaciers in Sinks Canyon and North Fork Canyon generally confirm earlier age estimates. Cosmogenic radionuclide (CRN, 10Be and 26Al) ages from moraines and striated bedrock surfaces previously mapped as Pinedale correspond to MIS2, while boulder exposure ages from moraines mapped as Bull Lake correspond generally to MIS5-MIS6. Geomorphic data from a moraine previously mapped as Younger pre-Sacagawea Ridge appears to correspond most closely to the Sacagawea Ridge glacial episode (MIS-16), but the uncertainty of a single 10Be exposure age suggests the unit could be as young as MIS-10 or as old as MIS-18. Boulders from a diamict on Table Mountain previously reported as Older pre-Sacagawea Ridge yield two 10Be exposure ages that suggest the presence of Early Pleistocene glacial activity here possibly older than 1–2 Ma (>MIS-30).Bedrock exposure ages within Sinks Canyon suggest the Pinedale valley glacier had retreated from the floor of Sinks Canyon to above PopoAgie Falls by ca. 15.3 ka. Cirque glaciers in Stough Basin appear to have retreated behind their riegels by ca. 16 ka, which suggests the cirque glaciers were decoupling across their riegels from the valley glaciers below at this time, prior to their readvance to form Lateglacial moraines.New 10Be boulder exposure ages from moraines previously correlated to the Temple Lake and Alice Lake allostratigraphic units in the cirques of Stough Basin and Cirque of the Towers show general equivalence to the stadial event just prior to the onset of the Bølling interstadial (17.5–14.7 ka) and to the Intra-Allerød Cold Period-Younger Dryas stadial phase (13.9–11.7 ka), respectively. From this evidence, the Temple Lake Alloformation of the Wind River Mountains now should correspond to the INTIMATE GS-2.1a (Oldest Dryas) stadial event while the Alice Lake Alloformation should correspond to the INTIMATE GS-2 stadial (IACP-Younger Dryas). Thus, we consider that evidence no longer exists for early-to mid-Holocene glacial events in the southern Wind River Range.

Empirical estimation of recreational exploitation of burbot, Lota lota, in the Wind River drainage of Wyoming using a multistate capture–recapture model

AbstractBurbot, Lota lota (Linnaeus), is a regionally popular sportfish in the Wind River drainage of Wyoming, USA, at the southern boundary of the range of the species. Recent declines in burbot abundances were hypothesised to be caused by overexploitation, entrainment in irrigation canals and habitat loss. This study addressed the overexploitation hypothesis using tagging data to generate reliable exploitation, abundance and density estimates from a multistate capture–recapture model that accounted for incomplete angler reporting and tag loss. Exploitation rate μ was variable among the study lakes and inversely correlated with density. Exploitation thresholds μ40 associated with population densities remaining above 40% of carrying capacity were generated to characterise risk of overharvest using exploitation and density estimates from tagging data and a logistic surplus‐production model parameterised with data from other burbot populations. Bull Lake (μ = 0.06, 95% CI: 0.03–0.11; μ40 = 0.18) and Torrey Lake (μ = 0.02, 95% CI: 0.00–0.11; μ40 = 0.18) had a low risk of overfishing, Upper Dinwoody Lake had intermediate risk (μ = 0.08, 95% CI: 0.02–0.32; μ40 = 0.18) and Lower Dinwoody Lake had high risk (μ = 0.32, 95% CI: 0.10–0.67; μ40 = 0.08). These exploitation and density estimates can be used to guide sustainable management of the Wind River drainage recreational burbot fishery and inform management of other burbot fisheries elsewhere.

New Feldspar Lead Isotope and Trace Element Evidence from the Sudbury Igneous Complex Indicate a Complex Origin of Associated Ni-Cu-PGE Mineralization Involving Underlying Country Rocks

The Sudbury Igneous Complex is the preserved remnant of a 1.85 Ga impact-induced crustal melt sheet that eventually crystallized into a coherent magmatic stratigraphy. At its base, the complex hosts world-class Ni-Cu- platinum group element (PGE) sulfide mineralization, distributed around the basin. Previous studies have shown strong Pb and Sr isotope contrasts between the North and South ranges of the Sudbury Igneous Complex. This study presents an extensive dataset of Pb isotopes and trace elements of hand-picked feldspars and sulfides from the complex, Ni-Cu-PGE mineralization, and underlying country rocks that reveals the finer details of isotopic decoupling between the North and South ranges. In terms of mean Pb isotope compositions, the new data confirm the previously established picture that North Range Main Mass rocks and associated Ni-Cu-PGE sulfide deposits are more juvenile than South Range Main Mass and associated mineralization, but there are several findings that point to a more complex evolution, with considerable implications for the source of sulfide in the Sudbury Igneous Complex. First, North Range sulfide ores and Main Mass silicates are more radiogenic than local footwall rocks. When expressed as per mil deviation in Pb-207/Pb-204 from a model contemporaneous mantle isochron (Delta Pb-207/Pb-204), the North Range Main Mass ores range from + 130 to + 421 and North Range ores range from + 160 to + 518 (primarily + 160 to + 260). This indicates that the Huronian Supergroup cover constituted a significant target rock type also in the North Range of the Sudbury Igneous Complex. Second, there is significant variability in the Pb isotope compositions between Ni-Cu-PGE sulfide deposits in both the North and South ranges and systematic differences between the average silicate compositions and sulfide compositions of Ni-Cu-PGE deposits. In the South Range Main Mass rocks, the Delta Pb-207/Pb-204 values range from + 360 to + 494, and associated ores range from + 150 to + 481 (primarily + 400 to + 480). Third, North Range mafic norite is much less radiogenic than North Range Main Mass, indicating that it may be contaminated by or derived from less radiogenic Ni-Cu-PGE-bearing mafic-ultramafic rocks. Finally, the Pb isotope variations between Sudbury Igneous Complex ores are corroborated by similar systematic variations in rare earth element patterns likely inherited by assimilation of local footwall rocks. Importantly, the considerable variations in the Pb isotope compositions of the ores between different deposits and the intermediate compositions between overlying Main Mass and footwall rocks suggest that the sulfides originally equilibrated with Main Mass, but acquired significant amounts of Pb (and therefore likely S and other metals) from local footwall rocks, most likely during thermomechanical erosion of the footwall rocks by dense molten sulfides. Potential S and metal sources are the Nipissing gabbros and the East Bull Lake intrusions, both of which are mineralized and whose Pb isotope compositions help to explain the variability in Sudbury Igneous Complex Ni-Cu-PGE sulfide ores. Small-scale lateral variations in Pb isotopes previously identified in the South Range of the Sudbury Igneous Complex are also present in the North Range. This indicates that the laterally extensive (200-km wide x 2.5-km thick) superheated Sudbury Igneous Complex melt sheet developed multiple chemically and isotopically isolated convection cells that continued to assimilate isotopically distinctive footwall rocks after impact. Lateral variations are much greater at lower levels in the magmatic stratigraphy than in the upper part of the stratigraphy. This resulted from a combination of three factors: (1) the overall depletion in Pb of the proto-Sudbury Igneous Complex (likely due to volatile element loss upon impact) made it more susceptible to contamination in this element than in more refractory elements; (2) assimilation of heterogeneous footwall into the base vs. incorporation of more homogeneous fallback breccias into the roof of the Sudbury Igneous Complex; and (3) possible hydrothermal roof alteration. Lead isotopes and trace element data for samples of the footwall collected and analyzed as part of this study used in tandem with data from previous work confirm that the Sudbury Igneous Complex can be reproduced by mixing the currently exposed target lithologies and indicate that no mantle or lower crustal component is required.

Carbonate and elemental accumulation rates in arid soils of mid-to-late Pleistocene outwash terraces, southeastern Wind River Range, Wyoming, USA

An important result of the past few decades of soil–geomorphic research is the awareness of the major role of dust and related carbonate precipitates in arid soils. The temporal evolution of soils developed on outwash terraces is still a matter of debate particularly regarding the type of evolution: progressive evolution and constant rates vs complex and/or interval-accumulation rates (progressive or regressive evolution possible). Soils on a chronosequence of outwash terraces just outside and downstream of canyon mouths of the Wind River Range (USA) were analysed for carbonate accumulation, organic carbon and poorly-crystalline pedogenetic oxyhydroxides stocks and pathways of chemical weathering. The soils and their carbonate stocks in the fine earth fraction (<2mm) can be used as a relative dating tool and appear to correlate with regional glacial chronostratigraphy [Pinedale (c. 20ky), Bull Lake (with two different units around 160 and 260ky) and Sacagawea Ridge (c. 660ky)]. Carbonate stocks of soils on these terraces increase with increasing soil age, but at generally decreasing rates. A quasi-steady state situation, however, was not reached before 1My. The time-split approach showed that CaCO3 accumulation rates were the highest during marine isotope (MIS) cold stages (2.13gCaCO3/m2/y), presumably due to a sparse vegetation cover during MIS2 (Pinedale) and 8 (lower Bull lake). Compared to organic carbon (c. 8–10kgCorg/m2), the inorganic carbon stocks of the fine earth fraction were much higher (c. 60kgCinorg/m2). Although the dominant pedogenic process here appears to be carbonate accumulation, chemical weathering and elemental leaching also are significant. Weathering data were obtained using immobile elements and rare earth elements (REE) as tracers. Chemical weathering and related leaching were particularly notable for Na originating from plagioclase. In addition, pedogenetic Al and Mn oxyhydroxides accumulate (220 and 32g/m2, respectively, after 660ky). These form at relatively low rates, however, when compared to moister and cooler conditions at higher altitudes in the Wind River Range. Due to the dry climate under which the terrace soils formed (annual precipitation 32cm/yr−1), weakly crystalline and amorphous Fe-oxyhydroxide is rapidly transformed into crystalline forms (hematite) over time. Consequently, the stocks of weakly crystalline and amorphous Fe-oxyhydroxide was significantly lower in the ~660ky soils (50g/m2) than in the Holocene soils (1000g/m2).The development of soils on Quaternary outwash terraces in this region is controlled through the rates of aeolian influx over time as modified by glacial/interglacial climate variations rather than by moisture availability. Consequently, carbonate accumulation rates in these soils are ‘influx limited’ rather than ‘moisture limited’.

Numerical modeling of the Snowmass Creek paleoglacier, Colorado, and climate in the Rocky Mountains during the Bull Lake glaciation (MIS 6)

AbstractWell-preserved moraines from the penultimate, or Bull Lake, glaciation of Snowmass Creek Valley in the Elk Range of Colorado (USA) present an opportunity to examine the character of the high-altitude climate in the Rocky Mountains during Marine Oxygen Isotope Stage 6. This study employs a 2-D coupled mass/energy balance and flow model to assess the magnitudes of temperature and precipitation change that could have sustained the glacier in mass-balance equilibrium at its maximum extent during the Bull Lake glaciation. Variable substrate effects on glacier flow and ice thickness make the modeling somewhat more complex than in geologically simpler settings. Model results indicate that a temperature depression of about 6.7°C compared with the present (1971–2000 AD) would have been necessary to sustain the Snowmass Creek glacier in mass-balance equilibrium during the Bull Lake glaciation, assuming no change in precipitation amount or seasonality. A 50% increase or decrease from modern precipitation would have been coupled with 5.2°C and 9.1°C Bull Lake temperature depressions respectively. Uncertainty in these modeled temperature depressions is about 1°C.

Estimates of Glacier Mass Loss and Contribution to Streamflow in the Wind River Range in Wyoming: Case Study

The Wind River Range is a continuous mountain range, approximately 160 km in length, in west-central Wyoming. The presence of glaciers results in meltwater contributions to streamflow during the late summer (July, August, and September: JAS) when snowmelt is decreasing; temperatures are high; precipitation is low; evaporation rates are high; and municipal, industrial, and irrigation water are at peak demands. Thus, the quantification of glacier meltwater (e.g., volume and mass) contributions to late summer/early fall streamflow is important, given that this resource is dwindling owing to glacier recession. The current research expands upon previous research efforts and identifies two glaciated watersheds, one on the east slope (Bull Lake Creek) and one on the west slope (Green River) of the Wind River Range, in which unimpaired streamflow is available from 1966 to 2006. Glaciers were delineated within each watershed and area estimates (with error) were obtained for the years 1966, 1989, and 2006. Glacier volume (mass) loss (with error) was estimated by using empirically based volume–area scaling relationships. For 1966 to 2006, glacier mass contributions to JAS streamflow on the east slope were approximately 8%, whereas those on the west slope were approximately 2%. The volume–area scaling glacier mass estimates compared favorably with measured (stereo pair remote sensed data) estimates of glacier mass change for three glaciers (Teton, Middle Teton, and Teepe) in the nearby Teton Range and one glacier (Dinwoody) in the Wind River Range.

Surface reconstruction and derivation of erosion rates over several glaciations (1 Ma) in an alpine setting (Sinks Canyon, Wyoming, USA)

At middle to high latitudes, many alpine valleys have been shaped by glaciers associated with periods of Pleistocene glaciation. Present glaciated valleys are characterised by broadened valley floors and U-shaped cross sections, continuously formed by glacial activity from initially V-shaped, fluvial cross sections. Sinks Canyon (Wind River Range, USA) is a glaciated valley characterised by a typical U-shaped cross section, containing till from several glacial advances over a range of at least 1Ma. The morphostratigraphic records indicate a fourfold difference in ice surface elevation between the youngest and oldest glacial periods, which is not easily explained by the present-day valley topography. To assess possible evolution scenarios of Sinks Canyon, we modelled the palaeovalley topography using a geographic information system (GIS) filtering technique in combination with temporal reference points from relative and numerically dated glacial deposits. Ice thicknesses were calculated using the shallow ice approximation.In our model, the valley became shallower and the topography smoother with increasing years back in time. The results suggest that valley topography with ages between 640 and 1000ka can clearly be distinguished from the present-day topography. Surfaces with ages of 130–200ka (attributable to MIS 6; Bull Lake glaciation) still could be discerned from present-day topography, but with relatively high uncertainties. The method did not work for topography less than ~100ka or older than ~1Ma. Erosion depths were calculated using the differences between present-day elevation and the modelled surfaces. Calculated erosion rates were within the range of reference values for glacial erosion (0.001 to 1mm a−1). Glacial erosion appears to have removed 0.52 to 0.72mm a−1 of rock within a time frame of 1Ma, assuming 200ka of aggregated glacial flow. If the glacial occupation was longer or the impact of fluvial erosion was not negligible (as assumed), then the calculated rate is lower. If the model assumes a less extended time of glacial flow (<100ka), the rate exceeds 1mm a−1. The method used for surface reconstruction and erosion rates calculation is highly dependent on (i) the reliability of the temporal anchor points and (ii) the assumption of physical glacier conditions (basal shear stress).

Geochemistry and Mineralogy of Platinum Group Element Mineralization in the River Valley Intrusion, Ontario, Canada: A Model for Early-Stage Sulfur Saturation and Multistage Emplacement and the Implications for "Contact-Type" Ni-Cu-PGE Sulfide Mineralization

The River Valley intrusion within the ~2.48 Ga East Bull Lake intrusive suite in Ontario, Canada, is an example of a mafic igneous intrusion with “contact-type” Ni-Cu-PGE sulfide mineralization along its base. Whereas many contact-type deposits are thought to form from in situ contamination of the magma by the addition of crustal S during emplacement, there are some intrusions, including the River Valley intrusion, which appear to have a much more complex history where the timing of S saturation, and thus the critical ore genesis processes, may have occurred much earlier, prior to emplacement. The River Valley intrusion is made up of a basal ~100 m of unlayered, inclusion-bearing units, overlain by layered cumulates. The basal units contain autoliths of gabbroic rocks and inclusions of footwall gneiss and amphibolites, all within a gabbroic matrix. Platinum-group element-rich magmatic sulfide mineralization occurs throughout both the inclusions and the matrix as blebby and disseminated sulfides. The matrix and inclusions can be separated into two distinct textural types: hydrothermally altered greenschist assemblages and unaltered metamorphic amphibolite assemblages. The platinum group mineral (PGM) assemblages vary only between textural types, and not between inclusions and matrix, being dominated tellurides in all rock types. The hydrothermally altered rocks, however, have fewer tellurides and an increased amount of Sb- and As-bearing PGM, indicative of minor fluid interaction, although the PGM have not been mobilized significantly away from the base metal sulfides. Precious and base metal geochemistry shows all rock types to have an excellent correlation between all the platinum group elements (PGE), indicating the presence of a single, well homogenized, PGE-rich sulfide liquid. However, Au and Cu appear to be decoupled from the PGE at low concentrations, although correlate well with each other, which is interpreted to be due to minor fluid redistribution and alteration of sulfide bleb margins. The overlying Layered units above the mineralized units are not PGE depleted. Trace element data, including (Th/Yb)PM and (Nb/Th)PM ratios, demonstrate that all River Valley rocks were formed from crustally contaminated magmas following interaction with local country rocks in a deeper subchamber; although some samples have S/Se ratios indicative of crustal S, most have S/Se ratios lower than the mantle range, indicative of S loss. We propose a multistage model for the formation of the mineralization in the River Valley intrusion with a major contamination event at depth with the addition of S from local crustal rocks, inducing sulfide saturation. Sulfide droplets were then enriched in PGE within a conduit system with possible further upgrading of sulfide metal tenors (and reduction of S/Se ratios) via partial dissolution of sulfide. The PGE-enriched sulfide liquid then settled in a staging chamber and partially crystallized before a major pulse of magma entrained sulfide liquid, eroded blocks of precrystallized and mineralized gabbro and footwall rocks, and emplaced an inclusion-bearing package as the lower 100 m or so of the River Valley intrusion. Later emplacement of main River Valley magma was from an S-undersaturated, PGE-fertile magma. The River Valley intrusion is thus an example whereby contact-type mineralization is purely a function of the earliest magma intruded containing preformed sulfide mineralization, rather than contamination triggering sulfide saturation in situ. In such cases, processes at depth determine the generation and subsequent tenor of the mineralization. In particular, dissolution of the sulfide can upgrade metal tenor, but subsequently will reduce S/Se ratios, masking the signature of crustal contamination. In addition, a multistage emplacement such as this will not necessarily preserve the characteristic increase in Cu/Pd ratios in the overlying cumulates that is often used in exploration for PGE deposits in mafic intrusions. Thus, a full understanding of all the field, geochemical, and mineralogical factors is required to construct genetic models for such deposits and especially in the interpretation of S/Se and Cu/Pd ratios as an indicator of crustal contamination and the presence of PGE mineralization.

The geochemistry and petrogenesis of the Blue Draw Metagabbro

The Blue Draw Metagabbro (BDM) in western South Dakota, is an 800m thick, layered intrusion, which is interpreted to have been intruded as a rift-related subvolcanic sill during the Palaeoproterozoic. The age and tectonic setting of the BDM are similar to those recorded by the East Bull Lake Suite of layered intrusions in Southern Ontario. These similarities have led previous authors to suggest that the two sets of intrusions are cogenetic. The East Bull Lake Suite intrusions are relatively well studied and are known to host significant contact-type Ni–Cu–PGE sulphide mineralisation, however, prior to this work, relatively little was known of BDM both in terms of its geochemistry and mineralisation potential. Chemostratigraphic profiles through the BDM show that the intrusion is the product of at least two magma pulses which fractionated to produce a sequence of rocks which grade from peridotitic at the base of the intrusion to gabbronoritic at the upper margin. Closed-system fractionation following the intrusion of the second magma pulse caused the magma to become saturated in sulphur and precipitate Ni–Cu–PGE bearing sulphides—now preserved in a low-grade 50m thick zone near the top of the intrusion. Petrological modelling shows that the parental melt of the BDM was a low-Ti tholeiite, with a trace element chemistry defined by enrichments in large-ion lithophile and light rare-earth elements and prominent negative Nb, Ta, and Ti anomalies. This ‘arc-like’ geochemistry recorded by the BDM parent magma is shared with parent magmas of the East Bull Lake Suite and may suggest that the BDM and its potential Canadian relatives share a common magmatic source. However, the relative ubiquity of such geochemical signatures in Archaean–Palaeoproterozoic intracontinental magmatic rocks coeval with the BDM suggests that the geochemical similarities recorded by the BDM and East Bull Lake Suite are non-unique and hence, are not definitive evidence of a genetic link between the two sets of intrusions. Instead, this geochemical signature that is common to many ancient igneous provinces may indicate the presence of a transient and (currently) poorly understood Archaean–Palaeoproterozoic mantle reservoir which was a globally significant magma source.

Magnetic analyses of soils from the Wind River Range, Wyoming, constrain rates and pathways of magnetic enhancement for soils from semiarid climates

In order to constrain the rate of magnetic enhancement in soils, we investigated modern soils from five fluvial terraces in the eastern Wind River Range, Wyoming. Profiles up to 1.2 m deep were sampled in 5-cm intervals from hand-dug pits or natural riverbank exposures. Soils formed in fluvial terraces correlated to the Sacajawea Ridge (730–610 ka BP), Bull Lake (130–100 ka BP) and Pinedale-age (∼20 ka BP) glacial advances. One soil profile formed in Holocene-age sediment. Abundance, mineralogy, and grain size of magnetic minerals were estimated through magnetic measurements. Magnetic enhancement of the A-horizon as well as an increase in fine-grained magnetic minerals occurred mostly in Bull Lake profiles but was absent from the older profile. Such low rates of magnetic enhancement may limit the temporal resolution of paleosol-based paleoclimate reconstructions in semiarid regions even where high sedimentation rates result in multiple paleosols. A loss of ferrimagnetic and an increase in antiferromagnetic minerals occurred with age. Our findings suggest either the conversion of ferrimagnetic minerals to weakly magnetic hematite with progressing soil age, or the presence of ferrimagnetic minerals as an intermediate product of pedogenesis. Absolute and relative hematite abundance increase with age, making both useful proxies for soil age and the dating of regional glacial deposits. All coercivity proxies are consistent with each other, which suggests that observed changes in HIRM and S-ratio are representative of real changes in hematite abundance rather than shifts in coercivity distributions, even though the modified L-ratio varies widely.

Glacier Impacts on Summer Streamflow in the Wind River Range, Wyoming

AbstractThe Wind River Range (WRR) of Wyoming is host to approximately 63 glaciers. Extensive research has been conducted using remote imagery to estimate the recent area and volume changes of these glaciers with the goal of estimating the potential effects of these changes on watershed streamflow. Results show that the glaciers were mostly in recession since 1966, the beginning of the study period. The current research was performed to supplement results from the remote imagery analyses. In this paper, streamflows from glaciated and nonglaciated watersheds in the WRR for the period 1967–1992 were analyzed. The difference in July-August-September (JAS) watershed flow magnitude for the 26-year period between glaciated (Green River and Bull Lake Creek) and nonglaciated (East Fork River and Wind River) watersheds ranged between 8 and 23%. As expected, the effects of glaciers on local streamflows during JAS were shown to be much greater than that of ice melt alone. The influence of glaciers accounted for 23–5...